Theoretical analysis of a push-pull fiber-optic hydrophone

A push-pull fiber-optic hydrophone is analyzed theoretically by closed-form approximations. The sensor is a fiber-optic Mach-Zehnder interferometer with a single-mode optical fiber from each arm wound on a separate tube. The two tubes are coaxial, with one tube inside the other. The acoustic medium is applied to the inside (outside) of the smaller (larger) diameter tube with the fiber wound on the tube's opposite side. This configuration is desired so that the acoustic signal modulates the optical phase of the arms with equal magnitude 180° out of phase. This gives the sensor twice the sensitivity of an analogous single tube sensor. The heat sinked interferometer arms with equal and opposite sensitivities are in relative close proximity to each other. This promotes good noise rejection to achieve a low minimum detectable acoustic pressure due to the interferometer's common-mode rejection.